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WA crabs feeling the pinch

WA crabs feeling the pinch

They’re easy to catch, yummy to eat, and when they’re young, they look like that alien from the Alien movies.

WA crabs feeling the pinch

I’m talking about the blue swimmer crab, one of WA’s most popular species to catch and eat.

Over the past decade, there have been concerns about low stocks, which has led to fishing restrictions in Cockburn Sound and the Peel-Harvey Estuary and continued monitoring and management.

The good news is scientists have been working on growing these crabs to help increase the population in local waters.

Getting crabs out of hot water

The Australian Centre for Applied Aquaculture Research (ACAAR) at South Metropolitan TAFE received a grant from the Recreational Fishing Initiatives Fund (RFIF) to identify future restocking options for blue swimmer crabs.

Culturing and stocking of these crabs hasn’t been done before in WA. If it works, it could result in a boost for crab populations in the systems such as Peel-Harvey and Swan Canning Estuaries.

Michael Tropiano, Habitat Officer at Recfishwest, says while each female can produce between 180,000 and 2 million eggs a season, very few of these eggs make it to adulthood due to predators.

“Getting them through the early life stages in a hatchery will give them a great start to life and a much better chance of survival,” he says.

‘Berried’ crabs—those already carrying eggs—were collected from local waters with help from the Department of Primary Industries and Regional Development (DPIRD) and taken to the ACAAR hatchery.

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Late-stage berried female crab being transferred to spawning tanks

Here, the crabs release the eggs naturally, and once they hatch, their life cycle begins.

From little things, big things grow

The first stage is the zoea, which Michael says resembles something more like the alien in the Alien movies.

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Day 10 zoea with ‘beak’, eyes stalked, six segments in abdomen

Just 1 – 2mm in size, the zoea can’t move much and is most at risk of predation in the wild, especially by filter feeders—animals like whales, sponges and corals—that feed on particles or small organisms strained out of water.

After 12 days, the zoea moults and changes into a megalopa, which is able to swim about.

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Day 12 megalopa with biting claws and able to swim freely

At around 19 days, the crablet develops, but is still only 2 – 3mm in size.

This is the stage when the claws start to develop.

“The young crablets seem to be getting used to their new claws and enjoy spending a lot of time opening and closing them,” Michael says.

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Day 19 crablet is only 2 – 3mm in size

“Even though the crablets are separated by size, trying out their new claws and their general aggressive nature means they will often cause damage to each other.”

Michael says one thing to be further trialled is to put some additional habitat in the hatchery tanks so they can hide and get some separation and avoid being ‘clawed’.

Crabbing is a popular pastime in WA but there have been concerns about low crab stocks. Scientists are working on growing these crabs to increase numbers.
Blue swimmer crabs can grow up to 25cm wide and have a claw span of up to 80 centimetres.

The tides are beginning to turn

In a WA first, despite the challenges, the project recently released a total of 3700 crablets into the Peel-Harvey Estuary.

The task of monitoring the success of stocking activities presents its own unique challenges because crabs moult.

Tagging crabs with traditional external yellow plastic tags is difficult, so the project is also investigating new internal tags that could involve the use of non-toxic dyes to colour organs inside the crab.

A second project has begun taking learnings from the first. It is hoped around 100,000 crablets will be released in metro waters in the second quarter of 2018. Both projects are supported by DPIRD and Recfishwest.

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